Substitute materials for glass, for the production for example of sheets, windows, spectacle lenses, optical lenses or automotive and aircraft glazing elements, have for a number of years increasingly been realized is using transparent plastics. Plastics based on polycarbonates and polymethyl methacrylates, in particular, have become established.
The use of plastics of this kind is of interest particularly for the automobile section, since by means of the plastics it is possible to produce shapes which, using glass as a material of construction, are realizable not at all or only at unacceptably high expense. For example, polycarbonate has rapidly become established for the “glazing” of automobile headlights.
In addition to their easy deformability, the advantage of plastics such as polycarbonate lies not least in the fact that the plastics have a much lower specific density than glass, and accordingly, for example, the vehicle weight can be markedly reduced.
Polycarbonate, furthermore, is more ductile than glass and is therefore better able to absorb impacts from stones by dissipation of energy. A disadvantage of plastics in comparison to glass, however, is the lower level of hardness. The result of this is a poorer resistance to external mechanical damage and hence a lower scratch resistance and a lower abrasion resistance. Particularly for the use of such materials in highly challenging environments, therefore, it is necessary to protect them from abrasion and scratching.
Specifically in the sector of automobile glazing, however, there are exacting requirements, which are specified in the standards ECE 43, ASTM 1044, and ASTM 1003.
For the coating of polycarbonate with a scratch-resistant coat which exhibits excellent results in the abrasion test (tested by means of the Taber test) it is possible to use polymer systems with a variety of compositions.
Sol-gel systems are used variously for this purpose, sometimes by means of plasma processes. These systems are frequently prepared from modified silanes and alkoxides by means of hydrolysis and condensation processes. Owing to the reactivity of these systems, the stability of the paint mixture on storage is usually very short. Moreover, the stability in such systems usually makes it more difficult to set higher solids contents.
In addition, radiation-curable coating compositions are also used to coat polycarbonate substrates. Thus U.S. Pat. No. 6,420,451 describes radiation-curable coating compositions for coating optical substrates, lenses for example, comprising various plastics substrates, such as polycarbonate, for example. Besides monofunctional acrylates, the coating compositions comprise urethane acrylates and colloidal metal oxides, especially SiO2 particles. As a result of their metal oxide particle content, these coating compositions lead to coatings featuring improved scratch resistance, and also exhibit effective adhesion to various substrates, and high compatibility with antireflection coatings in the case of the coating of optical lenses. In order to ensure transparency it is generally necessary to select a small particle size as compared with the wavelength of visible light, and so nanoscale metal oxides are used in these coating compositions that have small particle sizes, between 2 and 60 nm, preferably between 5 and 50 nm. Such nanoparticles are generally produced wet-chemically and in terms of price are ranked above silicon dioxide particles prepared by flame pyrolysis.
Commercial nanoparticles based on pyrogenically prepared (fumed) silica, in contrast, are substantially more favorable in price, and so it is entirely desirable, in coating compositions for coating polycarbonate substrates, to replace the special nanoparticles used, for example, in U.S. Pat. No. 6,420,451 by nanoparticles of that type that are based on pyrogenically prepared silica. The corresponding nanoparticles, however, is usually have substantially greater average particle sizes and therefore in general do not lead to transparent coatings.
Furthermore, WO06/028518 describes UV-curable coating compositions which comprise at least one radiation-curable (meth)acrylate, inorganic particles with a size of between 1 and 1000 nm, and optionally reactive diluents. Preferred particles are surface-modified SiO2 nanoparticles with a size of 5 to 80 nm, more particularly smaller than 50 nm, and with a narrow particle size distribution. These coating compositions are used for producing coatings featuring enhanced abrasion resistance, especially plastics substrates, such as road reflectors, for example, or coatings with enhanced stability, such as the coating of filter papers, for oil filters, for example. Polycarbonate substrates, however, are not described in WO06/028518.
As already stated, however, coating compositions suitable for coating polycarbonate must not only be curable to give scratch-resistant coatings but must also, furthermore, lead to transparent coatings having very good optical properties. Indications as to how the transparency—required in addition to the scratch resistance—of the coated substrates might be achieved are absent as such from WO06/028518.
Furthermore, DE 10 2006 020 987 A1 discloses dispersions of pyrogenically prepared silicon dioxide in organic solvent and their use in coating materials, especially clearcoat materials. The silicon dioxide dispersions described therein have the advantage that the clearcoat materials comprising these dispersions have a lower gray haze (lower haze values, the haze value being the fraction in % of light which is scattered by more than 2.5° from the axis of incidence) than clearcoat materials with conventional silicon dioxide dispersions. This is achieved by a silicon dioxide particle coarse fraction that is reduced in comparison to that of conventional silicon dioxide dispersions. The dispersions described therein comprise silicon dioxide particles in which 20% to 98% by weight, preferably 60% to 95% by weight, of the particles have a size of between 10 nm and 1500 nm and 2% to 80% by weight, preferably 5% to 40% by weight, of the particles have a size of between 1500 nm and 4000 nm.
Nevertheless, the haze determined here, which is sufficient for conventional clearcoat applications, is still not sufficient for the optical quality required in transparent coatings for polycarbonates. Here, less than 1% of the light ought to be scattered in the original, unexposed coating; in other words, the coatings ought to have a haze value <1 directly after their production, in other words prior to exposures—such as scratch exposures, for example.
The problem addressed by the present invention, therefore, was that of providing coating compositions which are suitable for coating polycarbonate substrates and which lead to cured coatings having a high transparency, low gray haze (haze values less than 1, determined using the BYK-Gardner Hazemeter AT-4727 instrument), and good scratch and abrasion resistance. The coating compositions ought further to be easy to handle and exhibit good processing viscosity and good flow.